Holes tilt angle measurement using FIB diagonal cut

ABSTRACT

A method of evaluating a region of a sample that includes a plurality of holes, wherein the method includes: taking a first image of the region by scanning the region with a first charged particle beam; evaluating the first image to determine a first center-to-center distance between first and second holes in the plurality of holes; milling a diagonal cut in an area within the region that includes the second hole at an angle such that an upper surface of the sample in the milled area where the second hole is located is recessed with respect to an upper surface of the sample where the first hole is located; thereafter, taking a second image of the region by scanning the region with the first charged particle beam; evaluating the second image to determine a second center-to-center distance between first and second holes in the plurality of holes; and comparing the second center-to-center distance to the first center-to-center distance.

BACKGROUND OF THE INVENTION

In the study of electronic materials and processes for fabricating suchmaterials into an electronic structure, a specimen of the electronicstructure can be used for microscopic examination for purposes offailure analysis and device validation. For instance, a specimen such asa silicon, gallium nitride or other type of wafer that includes one ormore integrated circuits (ICs) or other electronic structures formedthereon can be milled and analyzed with a focused ion beam (FIB) and/orwith a scanning electron microscope (SEM) to study specificcharacteristics of the circuits or other structures formed on the wafer.

One characteristic of structures formed on a wafer that can lead todefects is a hole that is etched at angle rather than vertically asmight have been intended. For example, in a deep hole, such as a via fora VNAND device, even a slight unintended angle can result in a defectivedevice. To illustrate, reference is made to FIG. 1, which is asimplified cross-sectional view of a partially formed semiconductordevice 100 formed on a substrate 110. As shown in FIG. 1, substrate 110can have multiple alternating layers formed over it, such as layers 120and 130. A deep hole 140 can be etched through the layers to a feature150. As an example, the hole 140 can be a via filled with a metal orconductive material that provides an electrical connection to feature150. If, when hole 140 is etched, the hole is etched at a slightlytilted angle (e.g., angle α) such that a hole 160 is formed instead ofhole 140, the hole 160 may not contact feature 150 and any circuit thatwas intended to have an electrical path between via 140 and feature 150might be defective.

Additionally, FIG. 1A is just a cross-section of substrate 110 and thusonly shows the sample (semiconductor device 100) along two axis (e.g.,the X and Z axis). The angle that hole 160 is etched can be misalignedfrom an ideal vertical hole in either or both of the X and the Y axis.FIG. 1B is a simplified illustration of a surface 170 of substrate 110at which features 150 are formed. As shown in FIG. 1B, hole 160 can bemisaligned (as compared to the intended via 140) with structure 150 inboth the X and the Y directions. In some applications, given the depthof features 150 below an upper surface of the sample, etching a tiltedhole 160 as little as one degree off can result the hole beingmisaligned with feature 150 or to other structures within substrate 110by an undesirable distance.

It can be difficult to accurately detect tilted angle holes formed in astructure such as semiconductor device 100. Accordingly, improvements indetecting tilted holes are desirable.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the disclosure provide methods and a system for detectingtilted holes formed in a sample, such as a semiconductor wafer.Embodiments can evaluate a sample, that includes two or more holesetched into the sample and determine whether the holes are formed at atilted angle or formed vertically at a 90 degree angle to the sample'ssurface. According to some embodiments, a region of the samplecontaining the two or more holes can be imaged at an angle normal to thesurface of the sample with a scanning electron microscope and thenmilled along a diagonal cut with a focused ion beam. After the diagonalmilling, a second image of the region can be taken at the same normalangle and the two images compared to determine if the center-to-centerdistance of the adjunct holes varies with depth.

If the holes are etched perfectly vertically (i.e., at a 90 degree anglewith respect to the surface of the substrate), the center-to-centerdistance between adjacent holes will remain constant throughout themilled depth. If, on the other hand, such a process detects a differencein the center-to-center distances of holes etched at the surface of thesubstrate as compared to at the milled depth, the holes were etched at atilted, non-vertical angle (i.e., and angle other than 90 degrees) andsome embodiments can determine the actual angle that the tilted holeswere etched at.

In some embodiments a method of evaluating a region of a sample thatincludes a plurality of holes is provided and the method can include:taking a first image of the region of the sample that includes theplurality of holes by scanning the region with a first charged particlebeam; evaluating the first image to determine a first center-to-centerdistance between first and second holes in the plurality of holes;milling a diagonal cut in an area within the region that includes thesecond hole at an angle such that an upper surface of the sample in themilled area where the second hole is located is recessed with respect toan upper surface of the sample where the first hole is located;thereafter, taking a second image of the region of the sample thatincludes the first and second holes by scanning the region with thefirst charged particle beam; evaluating the second image to determine asecond center-to-center distance between first and second holes in theplurality of holes; and comparing the second center-to-center distanceto the first center-to-center distance.

In some embodiments a system is provided for evaluating a sample such asthat described above. The system can include a vacuum chamber; a samplesupport configured to hold a sample within the vacuum chamber during asample evaluation process; a scanning electron microscope (SEM) columnconfigured to direct a first charged particle beam into the vacuumchamber; a focused ion beam (FIB) column configured to direct a secondcharged particle beam into the vacuum chamber; a processor and a memorycoupled to the processor. The memory can include a plurality ofcomputer-readable instructions that, when executed by the processor,cause the system to: take a first image of the region of the sample thatincludes the plurality of holes by scanning the region with a firstcharged particle beam; evaluate the first image to determine a firstcenter-to-center distance between first and second holes in theplurality of holes; mill a diagonal cut in an area within the regionthat includes the second hole at an angle such that an upper surface ofthe sample in the milled area where the second hole is located isrecessed with respect to an upper surface of the sample where the firsthole is located; thereafter, take a second image of the region of thesample that includes the first and second holes by scanning the regionwith the first charged particle beam; evaluate the second image todetermine a second center-to-center distance between first and secondholes in the plurality of holes; and compare the second center-to-centerdistance to the first center-to-center distance.

Still additional embodiments pertain to a non-transitorycomputer-readable memory that stores instructions for evaluating aregion of a sample, such as the sample described above, by: taking afirst image of the region of the sample that includes the plurality ofholes by scanning the region with a first charged particle beam;evaluating the first image to determine a first center-to-centerdistance between first and second holes in the plurality of holes;milling a diagonal cut in an area within the region that includes thesecond hole at an angle such that an upper surface of the sample in themilled area where the second hole is located is recessed with respect toan upper surface of the sample where the first hole is located;thereafter, taking a second image of the region of the sample thatincludes the first and second holes by scanning the region with thefirst charged particle beam; evaluating the second image to determine asecond center-to-center distance between first and second holes in theplurality of holes; and comparing the second center-to-center distanceto the first center-to-center distance.

Various implementations of the embodiments described herein can includeone or more of the following features. The method can further includedetermining an angle β at which the holes are tilted. The method canfurther include rejecting the sample from a manufacturing line if theangle β is greater than a predetermined value. The first hole can beoutside the area of the region milled at an angle. The sample can bepositioned within in a vacuum chamber of an evaluation tool thatincludes a scanning electron microscope (SEM) column and a focused ionbeam (FIB) column and the first and second images can be captured withthe SEM column and the milling can be performed with the FIB column. Thesample can be a semiconductor wafer.

In some implementations the angle β at which the holes are tilted can bedetermined according to the following formula:

${\tan\;\beta} = {\tan\;{\alpha\left( {\frac{x}{\Delta\; x} + {\cos\;\theta}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andΔx is the difference in the exact positions of the second tilted holebefore and after the diagonal cut and θ is the angle between a firstimaginary line bisecting centers of the first and second holes in thefirst image and a second imaginary line bisecting a center of the secondhole as initially imaged and the center of the second hole in the secondimage.

In some implementations the plurality of holes can be etched at an angletilted in a first horizontal plane, the sample can be milled in thefirst horizontal plane and the angle β at which the holes are tilted canbe determined according to the following formula:

${\tan\;\beta} = {\tan\;{\alpha\left( {1 + \frac{x}{\Delta\; x}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andΔx is the difference in the center-to-center measurements of the secondimage and the first image.

To better understand the nature and advantages of the presentdisclosure, reference should be made to the following description andthe accompanying figures. It is to be understood, however, that each ofthe figures is provided for the purpose of illustration only and is notintended as a definition of the limits of the scope of the presentdisclosure. Also, as a general rule, and unless it is evident to thecontrary from the description, where elements in different figures useidentical reference numbers, the elements are generally either identicalor at least similar in function or purpose.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified cross-sectional illustration of a sample havinga hole formed through a portion of the sample at a tilted angle;

FIG. 1B is a simplified illustration of a surface of the sample depictedin FIG. 1A depicting a hole that is misaligned in both the X and the Ydirections with a feature formed at the surface of the sample;

FIG. 2 is simplified illustration of a sample evaluation systemaccording to some embodiments of the disclosure;

FIG. 3 is a flowchart depicting steps associated with a method ofevaluating a sample according to some embodiments of the disclosure;

FIG. 4A is a simplified cross-sectional view of a semiconductor waferhaving multiple holes etched therein;

FIG. 4B is a simplified cross-sectional view of the semiconductor wafershown in FIG. 4A after a diagonal cut has been made in a region that canbe evaluated according to some embodiments disclosed herein;

FIG. 5 is a simplified cross-sectional view of the semiconductor wafershown in FIG. 4B along with a mathematical representation of the regionbeing evaluated according to some embodiments;

FIG. 6 is a simplified illustration depicting the evaluation of a samplein accordance with some embodiments;

FIGS. 7A and 7B are simplified illustrations depicting the evaluation ofa sample in accordance with some embodiments; and

FIG. 8 is a simplified illustration of an area on a semiconductor waferthat can include holes that can be evaluated to determine if the holeswere etched at a tilted angle according to some embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the disclosure pertain to methods and systems fordetecting tilted holes formed in a sample, such as a semiconductorwafer. Embodiments can evaluate a sample, that includes two or moreholes (e.g., holes from an array of thousands or millions of equallyspaced holes intended to have the same profile) etched into the sampleand determine whether the holes are etched into the sample at a 90degree angle to the sample's surface or whether the holes were etchedinto the sample at a non-vertical, tilted angle. According to someembodiments, a region of the sample containing the two or more holes canbe imaged at an angle normal to the surface of the sample with ascanning electron microscope and then milled along a diagonal cut with afocused ion beam. After the diagonal milling, a second image of theregion can be taken at the same normal angle and the two images comparedto determine if the center-to-center distance of the holes varies withdepth.

If the holes are etched vertically (i.e., at a 90 degrees angle withrespect to the surface of the substrate), the center-to-center distancebetween adjacent holes will remain constant throughout the milled depth.If, on the other hand, such a process detects a difference in thecenter-to-center distances of holes etched at the surface of thesubstrate as compared to at the milled depth, the holes were etched at atilted, non-vertical angle (i.e., and angle other than 90 degrees) andsome embodiments can determine the actual angle that the tilted holeswere etched at.

Example Sample Evaluation System

In order to better understand and appreciate the disclosure, referenceis first made to FIG. 2, which is a simplified schematic illustration ofan evaluation system suitable for detecting tilted holes in accordancewith embodiments of the disclosure. Sample evaluation system 200 can beused for, among other operations, defect review and analysis ofstructures formed on semiconductor wafers.

System 200 can include a vacuum chamber 210 along with a scanningelectron microscope (SEM) column 220 and a focused ion beam (FIB) column230. A supporting element 250 (e.g., a sample support pedestal) cansupport a sample 255 (e.g., a semiconductor wafer) within chamber 210during a processing operation in which the sample 255 (sometimesreferred to herein as an “object” or a “specimen”) is subject to acharged particle beam from one of the FIB or SEM columns. Supportingelement 250 can also move the sample within vacuum chamber 210 betweenthe field of view of the two columns 220 and 230 as required forprocessing.

One or more gases can be delivered to a sample being processed by a gassupply unit 260 for certain operations. For simplicity of explanationgas supply unit 260 is illustrated in FIG. 2 as a nozzle, but it isnoted that gas supply unit 260 can include gas reservoirs, gas sources,valves, one or more inlets and one or more outlets, among otherelements. In some embodiments gas supply unit 260 can be configured todeliver gas to a sample in the area of the sample that is exposed to thescan pattern of the charged particle beam as opposed to delivering gasto an entire upper surface of the sample. For example, in someembodiments gas supply unit 260 has a nozzle diameter measured inhundreds of microns (e.g., between 400-500 microns) that is configuredto deliver gas directly to a relatively small portion of the sample'ssurface that encompasses the charged particle beam scan pattern. Invarious embodiments, a first gas supply unit 260 can be configured todeliver gas to a sample disposed under SEM column 220 and a second gassupply unit 260 can be configured to deliver gas to a sample disposedunder FIB column 230.

SEM column 220 and FIB column 230 are connected to vacuum chamber 210 sothat a charged particle beam generated by either one of the chargedparticle columns propagates through a vacuumed environment formed withinvacuum chamber 210 before impinging on sample 255. SEM column 220 cangenerate an image of a portion of sample 255 by illuminating the samplewith a charged particle beam, detecting particles emitted due to theillumination and generating charged particle images based on thedetected particles. FIB column 230 can mill (e.g., drill a hole in)sample 255 by irradiating the sample with one or more charged particlebeams to form a cross section and can also smooth the cross section. Thecross section can include one or more first portions of a first materialand one or more second portions of a second material. The cross sectioncan also include additional portions of other materials. Conveniently,the smoothing operation typically involves utilizing smalleracceleration voltages in relation to the milling of the sample.

The particle imaging and milling processes each typically includescanning a charged particle beam back-and-forth (e.g., in a raster scanpattern) at a constant rate across a particular area of the sample beingimaged or milled. One or more lenses (not shown) coupled to the chargedparticle column can implement the scan pattern as is known to those ofskill in the art. The area scanned is typically a very small fraction ofthe overall area of sample. For example, the sample can be asemiconductor wafer with a diameter of either 200 or 300 mm while eacharea scanned on the wafer can be a rectangular area having a widthand/or length measured in microns or tens of microns.

In some embodiments, in order to evaluate a sample 255 that includes twoor more holes etched into the sample, the sample can be milled along adiagonal cut with the FIB column 230 and then imaged in a top-down mode(i.e., at an angle normal to the surface of sample 255) with SEM column220. Such a process can detect a difference in the center-to-centerdistances of holes etched at an angle that would not be present if theholes were etched vertically at 90 degrees. Further details of detectingtilted holes using such techniques are described below with respect toFIGS. 3-8.

While not shown in either of FIG. 2, system 200 can include one or morecontrollers, processors or other hardware units that control theoperation of system 200 by executing computer instructions stored in oneor more computer-readable memories as would be known to persons ofordinary skill in the art. By way of example, the computer-readablememories can include a solid-state memory (such as a random accessmemory (RAM) and/or a read-only memory (ROM), which can be programmable,flash-updateable and/or the like), a disk drive, an optical storagedevice or similar non-transitory computer-readable storage mediums.

Milling a Sample Along a Diagonal Cut

As mentioned above, embodiments set forth in the disclosure can be usedto evaluate a sample, such as sample 255, that includes two or moreholes etched into the sample. The sample can be milled along a diagonalcut with a focused ion beam and then imaged at an angle normal to thesurface of the sample with a scanning electron microscope beam. Such aprocess can detect whether a difference in the center-to-centerdistances of holes exits from the surface of the substrate as comparedto a deeper portion of the substrate. If the holes are etched perfectlyvertical (i.e., at a 90 degrees angle with respect to the surface of thesubstrate), the center-to-center distance between adjacent holes willremain constant throughout the milled depth. If, on the other hand, sucha process detects a difference in the center-to-center distances ofholes etched at the surface of the substrate as compared to at themilled depth, embodiments can determine that the holes were etched at anon-vertical angle (i.e., and angle other than 90 degrees) and candetermine the actual angle the holes were etched at.

To illustrate, reference is made to FIG. 3, which is a flowchartillustrating steps associated with a method 300 according someembodiments of the disclosure, and to FIGS. 4A and 4B, which are asimplified cross-sectional views of a semiconductor wafer 400 subjectedto the steps of method 400. Semiconductor wafer 400 can include multipleholes, such as an array of thousands or millions of small feature size,high aspect ratio holes, formed through one of more layers 420 formedover a semiconductor substrate 410 in a region 425 that can be evaluatedaccording to embodiments of the disclosure. For illustrative purposesonly, semiconductor wafer 400 is shown as having two vertically etchedholes 440 and two holes 460 etched at an angle that results in anon-vertical “tilted”) hole. The inclusion of both holes 440 and 460shown in overlapping regions is for explanatory purposes only. A personof ordinary skill in the art will appreciate that semiconductor wafer400 will actually include only one of the set of holes 440 or the set ofholes 460 in the area 425 that can be evaluated in accordance with themethods disclosed herein. To further simplify the discussion below, attimes the holes etched into semiconductor wafer 400 are referred tocollectively below as “holes 440, 460”. Since the semiconductor wafer400 only includes one set of the holes 440 or one set of the holes 460,such a description is understood to refer to whichever set of holes agiven sample wafer 400 actually includes: holes 440 or holes 460.Embodiments described in this disclosure teach a method of determiningwhether the semiconductor wafer 400 includes vertically oriented holes,such as holes 440, or angled holes, such as holes 460.

Also shown in FIGS. 4A and 4B are structures 450 formed at a depthwithin the semiconductor wafer 400 that the holes 440, 460 are intendedto be formed directly over. As one non-limiting example, holes 440 canbe vias formed through one or more layers 420 of dielectric material andstructures 450 can be a portion of a memory cell or similar electronicdevice formed on substrate 410. Structures 450 need not be present inall samples and embodiments described in the disclosure can detectwhether holes 440, 460 are formed vertically or tilted irrespective ofwhether structures, such as structures 450, is formed beneath the holes.

An initial step of method 300 can include moving the wafer 400 under thefield-of-view of a scanning electron microscope, such as the SEM column220 shown in FIG. 2, and taking an initial image of wafer in the regionthat includes an array of holes 440, 460 (block 310) using a top-downmode in which the SEM beam is normal to the surface 430 of wafer 400.The initial image can be evaluated to determine the center-to-centerdistance between a pair of holes in the array using, for example, knownimage analysis techniques (block 320). In some embodiments, the pair ofholes for which the center-to-center distance is determined can beadjacent holes as illustrated in FIGS. 4A and 4B. In other embodiments,however, the holes need not be adjacent and can be separated by multipleother holes. As simplistic, non-limiting examples, in an array of holeshaving 5 columns of 100 holes (i.e., 100 rows), block 320 coulddetermine the center-to-center distance between a first hole at column1, row 10 and a second hole at column 1, row 40. Alternatively, block320 could determine the center-to-center distance between a first holeat column 1, row 10 and a second hole at column 5, row 10. As can beappreciated by a person of ordinary skill in the art, any two holes inthe array could be chosen when determining the center-to-center distancein block 320 provided holes having the same expected spacing betweenthem are chosen for comparison purposes in the later steps of method300.

Next, the semiconductor wafer 400 can be moved under the field-of-viewof a focused ion beam column, such as FIB column 230 shown in FIG. 3,and the region that includes the imaged holes can be milled with adiagonal cut 470 (block 330). As shown in FIG. 4B, diagonal cut 470 canbe made at an angle α such that the cut 470 starts in a region 425 aspaced slightly away from where a first of the holes 440, 460 is etchedand becomes deeper as the cut is extended into a region 425 b where asecond of the holes 440, 460 is etched.

After diagonal cut 470 is made, the semiconductor wafer 400 can be movedback under the field-of-view of the SEM column and a second image of thewafer in region 425 can be taken that includes the same holes imaged inblock 310 (block 340) using the same top-down mode in which the SEM beamis normal to the surface 430 of wafer 400 as used for the first image.The second image can be evaluated to determine the center-to-centerdistance, at the surface defined by diagonal cut 470, between the sametwo holes for which the center-to-center distance was determined inblock 320 using, for example, known the same image analysis techniquesused to determine the center-to-center distance of the holes in thefirst image (block 350).

Next, the second center-to-center distance (determined in block 350) canbe compared to the first center-to-center distance (determined in block320) to determine if there is a difference between the two measurementsand evaluate whether the holes are etched at an undesirable tilted angle(block 360). For example, as shown in FIG. 4B, if the holes formed inwafer 400 are tilted holes 460, the center-to-center distance betweenthe holes will increase with along the diagonal cut. Thus, the twomeasurements will differ from each other by a difference of Δx. If, onthe other hand, the center-to-center distance does not change alongdiagonal cut (i.e., Δx=0), the holes formed in wafer 400 are verticalholes etched into the wafer at a 90 degrees angle to surface 430. As canbe appreciated, diagonal cuts at higher angles will result in a deepercut in area 425 b and a larger Δx to be seen for a given tilted angle βof holes 460.

In addition to determining whether the holes etched in sample 400 arevertical holes 440 or tilted holes 460, in some embodiments the actualangle β of the holes can be calculated. As stated previously, if thethere is no difference in the center-to-center distance as measuredbetween blocks 320 and 350, in some instances it can be assumed that theholes are vertical, such as holes 440. If, on the other hand, there is adifference between the center-to-center distances as measured in blocks320 and 350, it can be concluded that the holes are tilted, such asholes 460 and some embodiments can calculate the precise angle β of tiltfor the holes (e.g., in block 360) using one of the techniques describedbelow.

On-Axis Hole Tilt Angle Measurements

For example, some embodiments block 330 can include additional sub-stepsto determine if the holes 440, 460 were etched at a tilted angle and thedirection of the tilt prior to milling the diagonal cut 470. Forexample, in some embodiments block 330 can include making an initialdiagonal cut to determine if the center-to-center distances between theholes pre-cut and after the cut changes, and if the distance doeschange, use imaging techniques to determine the direction that the holes460 are tilted. Once the direction is determined, a second diagonal cut(i.e., diagonal cut 470) can be formed in the region being evaluated inthe direction of the tilt. Then, such embodiments can calculate theangle β that the tilted holes 460 were etched at using the followingformula:

$\begin{matrix}{{\left. \begin{matrix}{{\tan\;\beta} = \frac{h}{\Delta\; x}} \\{{\tan\;\alpha} = \frac{h}{x + {\Delta\; x}}}\end{matrix} \right\}->{\tan\;\beta}} = \frac{\left( {x + {\Delta\; x}} \right)\tan\;\alpha}{\Delta\; x}} & (1)\end{matrix}$where β is the angle at which the holes are tilted, α is the angle ofdiagonal cut 470 (which is known and which is in the same horizontalplane as angle β), x is the center-to-center distance between the firstand second holes as measured in block 320, βx is the difference in thecenter-to-center measurements between blocks 320 and 350 as determinedin block 360, and h is the distance between the milled depth at whichthe second hole is imaged in block 360 compared to the depth at which itwas imaged in block 320. FIG. 5 is a simplified cross-sectional view ofthe semiconductor wafer shown in FIG. 4B along with a mathematicalrepresentation of the region being evaluated according to someembodiments and formula (1) above.

Formula (1) can be simplified to the following:

$\begin{matrix}{{\tan\;\beta} = {\tan\;{\alpha\left( {1 + \frac{x}{\Delta\; x}} \right)}}} & (2)\end{matrix}$The distance βx is depicted in the simplified illustration of FIG. 6with respect to two tilted holes 460. As shown in FIG. 6, thecenter-to-center distance between a pair of tilted holes 460 (a firsthole adjacent to region 425 a and a second hole in region 425 b)measured from a first image taken prior to a milling operation (e.g., asmeasured in block 320 from a first image of wafer 400) is shown as thedistance x between the centers 460 a of the two holes in the firstimage. FIG. 6 also shows the center 460 b of the hole in region 425 b asrepresented in a second image (e.g., as taken in block 340) from ofwafer 400 after the wafer is milled along diagonal cut 470) where thecenter of the tilted hole 460 in region 425 b has shifted (due to thedepth at which the second image was taken and the angle of the tiltedhole) from hole center 460 a to hole center 460 b.

Once the angle of tilt is calculated, it can be used (e.g., by themanufacturer that is fabricating electronic devices on semiconductorwafer 400) to determine whether or not the tilt angle is withinacceptable manufacturing tolerances or whether to reject wafer 400.

Off-Axis Hole Tilt Angle Measurements

In other embodiments, the angles α and β do not need to be in the samedirection and thus do not include additional sub-steps, such as thosedescribed above in conjunction with block 330. For example, FIG. 7A is asimplified illustration depicting a change in position of a hole inregion 425 b when comparing a first image of a pair of tilted holes 460(e.g., as measured in block 320 and shown by the distance x between thetwo hole centers 460 a) to a second image of the tilted holes 460 (e.g.,where the center of the hole in region 425 b has shifted to hole center460 b in a second image taken during block 340) when the diagonal cut470 is not in the same horizontal plane as a tilted hole 460. Instead,the direction of diagonal cut 470 and the direction of the tilted holescan differ by an unknown angle.

Such embodiments can still calculate the angle β that the tilted holes460 were etched at using the following formula:

$\begin{matrix}{{\left. \begin{matrix}{{\tan\;\beta} = \frac{h}{\Delta\; x}} \\{{\tan\;\alpha} = \frac{h}{x + d}} \\{d = {\Delta\;{x \cdot \cos}\;\theta}}\end{matrix} \right\}->{\tan\;\beta}} = \frac{\left( {x + d} \right)\tan\;\alpha}{\Delta\; x}} & (3)\end{matrix}$where β is the angle at which the holes are tilted, α is the angle ofdiagonal cut 470 (which is known but which is off axis from thedirection of angle β), x is the center-to-center distance between thefirst and second holes as measured in a first, pre-milled image, βx isthe difference in the exact positions of the tilted hole before andafter the diagonal cut, h is the distance between the milled depth atwhich the second hole is imaged in a second, post-milled image comparedto the depth at which it was imaged in the first image, and θ is theangle between a first imaginary line 480 bisecting the hole centers 460a of the two holes 460 as initially imaged and a second imaginary line482 bisecting the hole center 460 a of the hole in region 425 b asinitially imaged and hole center 460 b of the hole in region 425 b fromthe second image as shown in FIG. 7A.

Formula (3) can be simplified to:

$\begin{matrix}{{\tan\;\beta} = {\tan\;{\alpha\left( \frac{x + {\Delta\;{x \cdot \cos}\;\theta}}{\Delta\; x} \right)}}} & (4)\end{matrix}$

And, formula (4) can be simplified to:

$\begin{matrix}{{\tan\;\beta} = {\tan\;{\alpha\left( {\frac{x}{\Delta\; x} + {\cos\;\theta}} \right)}}} & (5)\end{matrix}$

Once the angle of tilt is calculated, it can be used (e.g., by themanufacturer that is fabricating electronic devices on semiconductorwafer 400) to determine whether or not the tilt angle is withinacceptable manufacturing tolerances or whether to reject wafer 400.

In some instances, there is a possibility that the center-to-centerdistance between the holes in the first and second images will notchange even though the holes were etched at an undesirable, tiltedangle. That is, given a specific combination of the angle θ and theshift Δx, the center-to-center distance might not change. To illustrate,reference is made to FIG. 7B, which is a simplified illustrationdepicting a change in position of a hole between pre-milled andpost-milled images of a pair of tilted holes 460 when the diagonal cut470 is not in the same horizontal plane as a tilted hole 460 and wherethe center-to-center distances between the holes in each image is thesame.

As shown in FIG. 7B, prior to being milled diagonally as discussedherein, a pair of tilted holes 460 are spaced apart by a distance X(e.g., as shown by the distance X between the hole centers 460 a of eachhole). After the diagonal milling process the center of the tilted holein region 425 b moves from position 460 a to position 460 b. Whileposition 460 b is spaced apart from position 460 a by a distance 4 x,the hole in region 425 b is actually the same distance X away from thehole adjacent to region 425 a. Thus, some embodiments can determine thatholes are etched at a tilted angle (block 360) if either thecenter-to-center distance between the holes changes or the direction(azimuth) between the holes changes. Said differently, embodiments candetermine that holes are etched vertically if both the center-to-centerdistance between the holes and the direction (azimuth) between the holesremains unchanged. Example of a Sample Having a Plurality of Holes

In order to provide context to some aspects of the embodiments set forthin the disclosure, reference is made to FIG. 8, which is a simplifiedillustration of an area on a semiconductor wafer that can includeadjacent holes that can be evaluated to determine if the holes wereetched at a tilted angle according to some embodiments. Specifically,FIG. 8 includes a top view of wafer 800 along with two expanded views ofspecific portions of wafer 800. Wafer 800 can be, for example, a 200 mmor 300 mm semiconductor wafer and can include multiple integratedcircuits 810 (fifty two in the example depicted) formed thereon. Theintegrated circuits 810 can be at an intermediate stage of fabricationand the delayering techniques described herein can be used to evaluateand analyze one or more regions 820 of the integrated circuits. Forexample, Expanded View A of FIG. 8 depicts multiple regions 820 of oneof the integrated circuits 810 that can be evaluated and analyzedaccording to the techniques described herein. Expanded View B depictsone of those regions 820 that includes an array 830 of holes formedtherein.

Embodiments of the disclosure can analyze and evaluate the holes inregion 820 by capturing a first SEM image of area 820, milling theregion 820 along a diagonal cut as discussed above and then taking asecond SEM image of area 820. The SEM images can, for example, be doneby scanning the SEM beam back and forth within the region according to araster pattern, such as scan pattern 850 depicted in a simplified formatin the Expanded View B of FIG. 8. The milling process can mill region820 by scanning the FIB beam back and forth within the region accordingto a similar raster pattern where the beam is scanned line-by-line froma beginning portion 850 a to an ending portion 850 b where each scanline is milled a little longer than the previous scan line such that themilled region 820 has an angle where the depth of the milled region isdeeper in the ending portion 850 b than the beginning portion 850 a.

Any reference in the specification above to a method should be appliedmutatis mutandis to a system capable of executing the method and shouldbe applied mutatis mutandis to a computer program product that storesinstructions that once executed result in the execution of the method.Similarly, any reference in the specification above to a system shouldbe applied mutatis mutandis to a method that may be executed by thesystem should be applied mutatis mutandis to a computer program productthat stores instructions that can be executed by the system; and anyreference in the specification to a computer program product should beapplied mutatis mutandis to a method that may be executed when executinginstructions stored in the computer program product and should beapplied mutandis to a system that is configured to executinginstructions stored in the computer program product.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the describedembodiments. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice the describedembodiments. For example, while several specific embodiments of thedisclosure described above use an example semiconductor wafer as thesample, embodiments of the disclosure can be employed to delayer othertypes of samples including nanostructures formed on substrates otherthan a semiconductor wafer. Thus, the foregoing descriptions of thespecific embodiments described herein are presented for purposes ofillustration and description. They are not target to be exhaustive or tolimit the embodiments to the precise forms disclosed. Also, whiledifferent embodiments of the disclosure were disclosed above, thespecific details of particular embodiments may be combined in anysuitable manner without departing from the spirit and scope ofembodiments of the disclosure.

Further, it will be apparent to one of ordinary skill in the art thatmany modifications and variations are possible in view of the aboveteachings. For example, while FIG. 3 depicted a specific order of stepsaccording to some embodiments, the order can be varied in otherembodiments. As one particular example, in some embodiments thecenter-to-center distances between adjacent holes in the first andsecond images can be determined at any appropriate time after the firstand second images are captured. As another example, in some embodiments,a single image can be used to determine the center-to-center distancesof the holes in both block 320 and block 350. For example, often holesformed in a sample are part of a large array of holes having the samespacing. A single image taken in block 340 can capture both holes in theregion that is milled and holes outside the milled region that are inthe same array of holes as those within the region. Since the spacing ofthe holes in the array is consistent, in some embodiments block 360 cancompare the center-to-center distance of holes within the region milledin block 330 to the center-to-center distance of holes outside themilled region.

Because the illustrated embodiments of the present disclosure may forthe most part, be implemented using electronic components and circuitsknown to those skilled in the art, details of such are not be explainedin any greater extent than that considered necessary as illustratedabove, for the understanding and appreciation of the underlying conceptsof the present disclosure and in order not to obfuscate or distract fromthe teachings of the present disclosure.

What is claimed is:
 1. A method of evaluating a region of a sample thatincludes a plurality of holes, the method comprising: taking a firstimage of the region of the sample that includes the plurality of holesby scanning the region with a first charged particle beam; evaluatingthe first image to determine a first center-to-center distance betweenfirst and second holes in the plurality of holes; milling a diagonal cutin an area within the region that includes the second hole at an anglesuch that an upper surface of the sample in the milled area where thesecond hole is located is recessed with respect to an upper surface ofthe sample where the first hole is located; thereafter, taking a secondimage of the region of the sample that includes the first and secondholes by scanning the region with the first charged particle beam;evaluating the second image to determine a second center-to-centerdistance between first and second holes in the plurality of holes; andcomparing the second center-to-center distance to the firstcenter-to-center distance.
 2. The method of claim 1 further comprisingdetermining an angle θ at which the holes are tilted.
 3. The method ofclaim 2 further comprising rejecting the sample from a manufacturingline if the angle θ is greater than a predetermined value.
 4. The methodof claim 1 further comprising determining an angle β at which the holesare tilted according to the following formula:${\tan\;\beta} = {\tan\;{\alpha\left( {\frac{x}{\Delta\; x} + {\cos\;\theta}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the exact positions of the second hole before andafter the diagonal cut and θ is the angle between a first imaginary linebisecting centers of the first and second holes in the first image and asecond imaginary line bisecting a center of the second hole as initiallyimaged and the center of the second hole in the second image.
 5. Themethod of claim 1 wherein the first hole is outside the area of theregion milled at an angle.
 6. The method of claim 1 wherein theplurality of holes are etched at an angle tilted in a first horizontalplane and the sample is milled in the first horizontal plane.
 7. Themethod of claim 6 further comprising determining an angle β at which theholes are tilted at which the plurality of holes are tilted according tothe following formula:${\tan\;\beta} = {\tan\;{\alpha\left( {1 + \frac{x}{\Delta\; x}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the center-to-center measurements of the secondimage and the first image.
 8. The method of claim 1 wherein the sampleis positioned within in a vacuum chamber of an evaluation tool thatincludes a scanning electron microscope (SEM) column and a focused ionbeam (FIB) column and the first and second images are taken with the SEMcolumn and the milling is performed with the FIB column.
 9. The methodof claim 8 wherein the sample is a semiconductor wafer.
 10. A system forevaluating a region of a sample that includes a plurality of holes, thesystem comprising: a vacuum chamber; a sample support configured to holda sample within the vacuum chamber during a sample evaluation process; ascanning electron microscope (SEM) column configured to direct a firstcharged particle beam into the vacuum chamber; a focused ion beam (FIB)column configured to direct a second charged particle beam into thevacuum chamber; a processor and a memory coupled to the processor, thememory including a plurality of computer-readable instructions that,when executed by the processor, cause the system to: take a first imageof the region of the sample that includes the plurality of holes byscanning the region with a first charged particle beam; evaluate thefirst image to determine a first center-to-center distance between firstand second holes in the plurality of holes; mill a diagonal cut in anarea within the region that includes the second hole at an angle suchthat an upper surface of the sample in the milled area where the secondhole is located is recessed with respect to an upper surface of thesample where the first hole is located; thereafter, take a second imageof the region of the sample that includes the first and second holes byscanning the region with the first charged particle beam; evaluate thesecond image to determine a second center-to-center distance betweenfirst and second holes in the plurality of holes; and compare the secondcenter-to-center distance to the first center-to-center distance. 11.The system for evaluating a sample set forth in claim 10 wherein theprocessor further causes the system to determine an angle θ at which theholes are tilted.
 12. The system for evaluating a sample set forth inclaim 10 wherein the first hole is outside the area of the region milledat an angle.
 13. The system for evaluating a sample set forth in claim10 wherein the processor further causes the system to determine an angleβ at which the holes are tilted according to the following formula:${\tan\;\beta} = {\tan\;{\alpha\left( {\frac{x}{\Delta\; x} + {\cos\;\theta}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the exact positions of the second hole before andafter the diagonal cut and θ is the angle between a first imaginary linebisecting centers of the first and second holes in the first image and asecond imaginary line bisecting a center of the second hole as initiallyimaged and the center of the second hole in the second image.
 14. Thesystem for evaluating a sample set forth in claim 10 wherein theplurality of holes are etched at an angle tilted in a first horizontalplane and the processor further causes the system to mill the sample inthe first horizontal plane.
 15. The system for evaluating a sample setforth in claim 14 wherein the processor further causes the system todetermine an angle β at which the holes are tilted at which theplurality of holes are tilted according to the following formula:${\tan\;\beta} = {\tan\;{\alpha\left( {1 + \frac{x}{\Delta\; x}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the center-to-center measurements of the secondimage and the first image.
 16. A non-transitory computer-readable memorythat stores instructions for evaluating a region of a sample thatincludes a plurality of holes by: taking a first image of the region ofthe sample that includes the plurality of holes by scanning the regionwith a first charged particle beam; evaluating the first image todetermine a first center-to-center distance between first and secondholes in the plurality of holes; milling a diagonal cut in an areawithin the region that includes the second hole at an angle such that anupper surface of the sample in the milled area where the second hole islocated is recessed with respect to an upper surface of the sample wherethe first hole is located; thereafter, taking a second image of theregion of the sample that includes the first and second holes byscanning the region with the first charged particle beam; evaluating thesecond image to determine a second center-to-center distance betweenfirst and second holes in the plurality of holes; and comparing thesecond center-to-center distance to the first center-to-center distance.17. The non-transitory computer-readable memory set forth in claim 16wherein the instructions for evaluating the region further includeinstructions to determine an angle β at which the holes are tilted. 18.The non-transitory computer-readable memory set forth in claim 16wherein the first hole is outside the area of the region milled at anangle.
 19. The non-transitory computer-readable memory set forth inclaim 16 wherein the instructions for evaluating the region furtherinclude instructions to determine an angle β at which the holes aretilted according to the following formula:${\tan\;\beta} = {\tan\;{\alpha\left( {\frac{x}{\Delta\; x} + {\cos\;\theta}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the exact positions of the second hole before andafter the diagonal cut and θ is the angle between a first imaginary linebisecting centers of the first and second holes in the first image and asecond imaginary line bisecting a center of the second hole as initiallyimaged and the center of the second hole in the second image.
 20. Thenon-transitory computer-readable memory set forth in claim 16 whereinthe plurality of holes are etched at an angle tilted in a firsthorizontal plane and the instructions for evaluating the region furtherinclude instructions to determine an angle β at which the holes aretilted at which the plurality of holes are tilted according to thefollowing formula:${\tan\;\beta} = {\tan\;{\alpha\left( {1 + \frac{x}{\Delta\; x}} \right)}}$where α is the angle of diagonal cut, x is the center-to-center distancebetween the first and second holes as measured in the first image, andβx is a difference in the center-to-center measurements of the secondimage and the first image.